Sulfur dioxide and/or oxides of nitrogen emissions and the products of their atmospheric conversion are significant hazards to human health, welfare, and safety. Their presence is associated with an estimated 25,000 deaths yearly, prolonged upper respiratory and lung damage, and aggravation of chronic respiratory ailments. Additional adverse effects of sulfur oxides pollution on man's welfare and safety encompass such serious environmental ecological consequences as reduced crop yields, inhibited forestation and fish growth, reduced sunlight, accelerated materials corrosion and restricted visibility.
Various procedures have been suggested in the prior art for removing contaminates such as sulfur dioxide from gas streams. For example, the use of alkaline agents such as limestone, lime, ammonia, or sodium hydroxide to remove SO.sub.2 from combustion gases occurs by the reactions: EQU Ca (OH).sub.2 + H.sub.2 O + SO.sub.2 .fwdarw. CaSO.sub.3 + H.sub.2 O EQU naOH + SO.sub.2 + H.sub.2 O .fwdarw. NaHSO.sub.3 + H.sub.2 O EQU naHSO.sub.3 + NaOH .fwdarw. Na.sub.2 SO.sub.3 + H.sub.2 O EQU (nh.sub.4)oh + so.sub.2 + h.sub.2 o .fwdarw. (nh.sub.4)hso.sub.3 + h.sub.2 o EQU (nh.sub.4)hso.sub.3 + nh.sub.4 oh .fwdarw. (nh.sub.4).sub.2 so.sub.3 + h.sub.2 o
the principal disadvantages of such means to remove sulfur dioxide from combustion gases are that the alkaline reagents must be purchased or otherwise acquired, usually at some cost, and that the resulting bisulfites which present a waste disposal problem are corrosive and of little or no value. Another disadvantage of utilizing these alkaline reagents is their lack of reactivity with oxides of nitrogen and, therefore, their inability to effectively remove them from the gas stream.
The use of both metallic or non-metallic sulfides has also been proposed for removing SO.sub.2 from gas streams. Frequently, however, the sulfides used have been relatively water soluble and basic. Thus, while these alkaline sulfides are effective to an extent in removing SO.sub.2, they often do so with formation of large amounts of bisulfites, which must be disposed of, and/or the evolution of hydrogen sulfide.
Iron sulfide slurry is uniquely appropriate for removing SO.sub.2 from gas streams, since its low solubility in water and stability in mildly acid environments make possible the reaction: EQU 4FeS + 3SO.sub.2 .fwdarw. 2 Fe.sub.2 O.sub.3 + 7S
without significant sulfite formation.
Although this procedure is advantageous where no re-use or re-cycling of the sulfur and Fe.sub.2 O.sub.3 product is contemplated, the high consumption of energy to regenerate FeS makes the process somewhat less efficient where such recycling is planned and efficiency of SO.sub.2 is somewhat diminished.
It is therefore an object of the present invention to provide a process for removing SO.sub.2 and NO.sub.x from gas streams which is economical and practical from the standpoint of both SO.sub.2 removal, materials used, products recovered and energy consumption and which precludes formation of quantities of unwanted waste products or residue.
According to the present invention, gases containing either sulfur dioxide, or nitrogen oxides, or both, are scrubbed with particulate ferrous sulfide in the presence of moisture such as an aqueous slurry at a controlled pH above 5.5 and preferably 5.5 to 7.5 and a temperature of ambient to 100.degree. C. to form sulfur-rich iron sulfides, iron disulfide and iron sulfate. The reaction is controlled with regard to duration of reaction, pH, temperature and proportion of FeS to SO.sub.2 so as not to proceed to the formation of elemental sulfur and iron oxide. The iron disulfide and sulfate are then dissociated to ferrous sulfide (FeS) and sulfur by heating them at a temperature of 650.degree. C. to 900.degree. C. in a dry, reducing atmosphere. If the dissociation of the iron disulfide and iron sulfate are carried out in an inert atmosphere, rather than a reducing atmosphere, formation of the intermediate product pyrrhotite occurs. The sulfur, which is vaporized upon dissociation, can be later recovered by condensation and the iron sulfide is recycled for further scrubbing. It is particularly important, however, that the reaction of SO.sub.2 or NO.sub.x with the ferrous sulfide during the scrubbing operation be terminated prior to the significant formation of elemental sulfur and Fe.sub.2 O.sub.3 since considerably more energy is needed to convert these compounds to ferrous sulfide and greater efficiency of SO.sub.2 removal is thereby realized.
The basic system chemistry between the reagent ferrous sulfide and the sulfur dioxide and nitrogen oxide gases is: EQU 3FeS + 2SO.sub.2 .fwdarw. FeSO.sub.4 + 2FeS.sub.2 EQU 4no + feS .fwdarw. 2N.sub.2 + FeSO.sub.4
nitric Oxide (NO) is used to represent nitrogen oxides since it typically comprises the bulk of the nitrogen oxides in combustion gases.
By virtue of the hydrolysis of ferrous sulfide in the scrubbing procedure, the pH during reaction with the SO.sub.2 and NO.sub.x will tend to maintain itself within the required range. Should the pH fall below about 5.5 however (thereby encouraging formation of Fe.sub.2 O.sub.3, bisulfites and some H.sub.2 S)an alkaline agent can be used to elevate the pH. Ordinarily the duration of the actual contact time in the scrubber will be in the order of about 1-8 seconds. In order to avoid having the reaction of the SO.sub.2 and FeS proceed to formation of Fe.sub.2 O.sub.3 and sulfur, it is also important that the mole ratio of FeS to SO.sub.2 be maintained greater than a minimum of 1.5 to 1 and preferably 3 to 1.
The transition from FeS to FeS.sub.2 passes through the representative iron sulfide compounds presented in Table 1. These intermediate compounds, as well as the final disulfide, are typical of the complex crystal lattice structures that iron sulfide is capable of forming. It is the ability to hydrolyze to buffer the pH in the proper range and also to form the intermediate sulfides and eventually the disulfide that make iron sulfide unique among sulfides such as the sulfides of alkali and alkaline earth elements.
Scrubbing of the SO.sub.2 and or NO.sub.x containing gas can be carried out using conventional scrubbing techniques as long as the proper conditions of temperature, pH and duration of reaction are observed. Ranges and typical conditions and procedures for this scrubbing are as follows:
Scrubber L/G ratio 35-150(100) gpm/1000scfm PA1 Inlet flue gas temp. 100.degree.-350.degree. C. (230.degree. C.) PA1 Outlet flue gas temp. 50.degree.-70.degree. C. (60.degree. C.) PA1 FeS Slurry temp. 50.degree.-70.degree. C. (60.degree. C.) PA1 FeS Slurry Concentration 2-6% PA1 FeS Slurry pH 5.5-7.5 PA1 Duration of reaction 1-8 second
Typical conditions for regeneration of the iron disulfide to ferrous sulfide are as follows:
TABLE 1 ______________________________________ Regeneration Temperature 650.degree.-850.degree. C (750.degree. C) Reducing Atmosphere Retention in Kiln 2-5 (3) minutes MINERAL FORMULA STRUCTURE ______________________________________ Mackinawite Fe.sub.9 S.sub.8 Tetrahedral Troilite FeS Hexagonal Pyrrhotite Fe.sub.11 S.sub.12 Fe.sub.10 S.sub.11 Hexagonal Fe.sub.9 S.sub.10 Pyrrhotite Fe.sub.7 S.sub.8 Monoclinic Smythite Fe.sub.9 S.sub.11 Rhombohedral Greigite Fe.sub.3 S.sub.4 Cubic Marcasite FeS.sub.2 Hexagonal Pyrite FeS.sub.2 Cubic ______________________________________